Promotion to captain is governed by Department of Defense policies derived from the Defense Officer Personnel Management Act of 1980. DOPMA guidelines suggest 95% of first lieutenants should be promoted to captain after serving a minimum of two years at their present rank.

An Army and Marine captain generally commands company-sized units. When given such a command, they bear the title company commander. Captains also instruct at service schools and combat training centers and are often staff officers at the battalion level. Marine captains also serve as officer selection officers, commanding recruiting stations for commissioned officers.

An Air Force captain's authority varies by group assignment. In an operations group, senior captains may be flight commanders while more junior captains may be heads of departments. In the maintenance or logistics and mission support groups they are almost always flight commanders. In the medical group, captains usually have limited administrative and command responsibility as captain is frequently the entry level rank for most medical officers and dental officers.[1]

[1]No universal insignia for officer candidate rank; Navy candidate insignia shown[2]Unofficial 1945 proposal for General of the Armies insignia; John J. Pershing's GAS insignia: ; George Dewey's AN insignia: [3]Rank used for specific officers during World War II and Korea only, not permanent addition to rank structure[4]Grade is authorized by the U.S. Code for use but has not been created[5]Grade has never been created or authorized